Membrane transport

2019.10.01

Dr. Ildiko Pecsi

Dept. of Biophysics, University of Pécs

The cellular organization

Biological membranes / cell membranes

Major functions of the cytoplasmic membrane

Semipermeable membrane- transport of different molecules

Anchor soluble proteins- transport, signaling, bioenergetics

Energy storage- generation of proton motive force

Biological membranes / cell membranes

Key functions:1. Barrier, semi-permeable lipid bilayer

2. Specific functions – integral or peripheralmembrane proteins

3. Regulation of the transport of molecules- aktive, passive, facilitated difussion, endo-exocytosis

4.Detection of signal molecules and transmission

5. Cell-cell communication

6. Cell movement

Biological membranes / cell membranes

Structure Phospholipid

Red blood cell

10 6 lipid molecule/ µm2

6-8

nm

Singer and Nicholson fluid mosaic model from1972

Constituents of membrane lipid bilayers

Amphiphilic molecules

Phospholipids (phosphoglycerides)• 3-carbon backbone of glycerol• 2 long- chain fatty acid esterified to C1 and C2 of the glycerol• The main constituent• Synthetized in the ER Sphingolipids• sphingosine+ fatty acid=ceramid• ceramid+phosphate+choline=sphingomyelins;• Synthetized in the Golgi • More abundant in the plasma membrane then in

the membranes inside cells

Constituents of membrane lipid bilayers

Glycolipids

• 3 types of glycolipids:• Sphingolipids• Glycerol glycolipids with sugar chains attached to the hydroxyl

on C3of diglycerides• Glycosylphosphatidylinositols (GPI)They are on the outer membrane surface and not incorporatedinto membrane bilayers.

Sterols

CholesterolThe rigid four ring structure is apolar so it inserts into the core of bilayers• regulation the fluidity of the membrane• temperature• short and/or unsaturated bonds- influence the physical properties ofmembranes

Lipid raft in the outer leaflet of the plasma membrane enriched in cholesterol and sphingolipids.

Lipid % composition in the hepatocyte rat liver cells of the different cellcompartments

Mobility within the membrane

Membranes with barrier functions

The lipid bilayer of cell membranes prevents the passage of most polar molecules

Large uncharged polar molecules and ions cant diffuse throughthe membrane and require transport

In general the smaller the molecule and the more hydrophobic or non polar it is, the more rapidly it will diffuse across the membrane bilayer

Membrane proteins main functions

Transport across membrane may occur eitheractively or passively

Some functions of membrane proteins

Potassium Channels

Integral proteins with a hydrophilic inner pore viawhich potassium ions may be transported

The channel is comprised of four transmembranesubunits, while the inner pore contains a selectivity filterat its narrowest region that restricts passage ofalternative ions

Potassium channels are typically voltage-gated andcycle between an opened and closed conformationdepending on the transmembrane voltage

Transport proteins

Uniport mechanism

Symport and antiport mechanisms

The sodium-potassium pump

Active transport

Example: Membrane potential

Glycocalyx (cell coat)